High efficiency photovoltaic cells

ABSTRACT

Novel structures of photovoltaic cells (also called as solar cells) are provided. The cells are based on nanoparticles or nanometer-scaled wires, tubes, and/or rods, which are made of electrical materials covering semiconductors, insulators, and also metallic in structure. These photovoltaic cells have large power generation capability per unit physical area over the conventional cells. These cells will have enormous applications such as in space, commercial, residential and industrial applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/766,575 filed Jan. 28, 2006, and also patent application Ser. No.11/626,826, filed on Jan. 24, 2007.

FIELD OF INVENTIONS

This patent specification relates to structures of photovoltaic cells(hereafter mentioned also as “solar cells”). More specifically, itrelates to photovoltaic cells comprising structures that increase thejunction area and absorb a broad solar spectral spectrum in order toincrease power generation capability per unit area. This also relates tophotovoltaic cells containing nano-scaled blocks. These photovoltaiccells can be used in commercial, residential, and also industrialapplications for power generation.

BACKGROUND OF THE INVENTIONS

Photovoltaic cells where light is converted into electric power to befed to external loads, which are electrically connected to thephotovoltaic cells, have been prevailing in a wide range of applicationssuch as consumer electronics, industrial electronics, and spaceexploration. In consumer electronics, photovoltaic cells that consist ofmaterials such as amorphous silicon are used for a variety ofinexpensive and low power applications. Typical conversion efficiency,i.e. the solar cell conversion efficiency, of amorphous silicon basedphotovoltaic cells is in the range of ˜10% [Yamamoto K, Yoshimi M,Suzuki T, Tawada Y, Okamoto T, Nakajima A. Thin film poly-Si solar cellon glass substrate fabricated at low temperature. Presented at MRSSpring Meeting, San Francisco, April 1998.]. Although the fabricationprocesses of amorphous silicon based photovoltaic cells are rathersimple and inexpensive, one notable downside of this type of cell is itsvulnerability to defect-induced degradation that decreases itsconversion efficiency.

In contrast, for more demanding applications such as residential andindustrial solar power generation systems, either poly-crystalline orsingle-crystalline silicon is typically used because there are morestringent requirements of better reliability and higher efficiency thanthose in consumer electronics. Photovoltaic cells consisting ofpoly-crystalline and single-crystalline silicon generally offerconversion efficiencies in the range of ˜20% and ˜25% [Zhao J, Wang A,Green M, Ferrazza F. Novel 19.8% efficient ‘honeycomb’ texturedmulticrystalline and 24.4% monocrystalline silicon solar cell. AppliedPhysics Letters 1998; 73: 1991-1993.] respectively. As many concernsassociated with a steep increase in the amount of the worldwide energyconsumption are raised, further development in industrial solar powergeneration systems has been recognized as a main focus for analternative energy source. However, due to the high cost ($3 to $5/Watt)of today's Si-based solar cell, it is not yet widely accepted as analternative energy source solution.

Group II-V compound semiconductors, for example CdTe and CdS, have beenconsidered for the purpose of creating industrial solar power generationsystems, manufactured at a lower cost and maintaining a moderateconversion efficiency. This approach resulted in a comparable conversionefficiency of ˜17% [Wu X, Keane J C, Dhere R G, DeHart C, Duda A,Cessert T A, Asher S, Levi D H, Sheldon P. 16.5%-efficient CdS/CdTepolycrystalline thin-film solar cell. Proceedings of the 17th EuropeanPhotovoltaic Solar Energy Conference, Munich, 22-26 Oct. 2001;995-1000.]. This conversion efficiency is comparable to those for thesingle crystalline silicon photovoltaic devises; however, the toxicnature of these materials is of great concern for environment.

Group I-III-VI compound semiconductors, such as CuInGaSe₂, have alsobeen extensively investigated for industrial solar power generationsystems. This material can potentially be synthesized at a much lowercost than its counterpart, single crystalline silicon. However, aconversion efficiency of ˜19%, is comparable to that of singlecrystalline silicon based cells and can be obtained, thus far, only bycombining with the group II-VI compound semiconductor cells [Contreras MA, Egaas B, Ramanathan K, Hiltner J, Swartzlander A, Hasoon F, Noufi R.Progress toward 20% efficiency in Cu(In,Ga)Se polycrystalline thin-filmsolar cell. Progress in Photovoltaics: Research and Applications 1999,7: 31-316.], which again raises issues associated with the toxic natureof these materials.

Photovoltaic cells designed for several exclusive applications, wherethe main focus is high conversion efficiency, generally consist of groupIII-V semiconductors, including GaInP and GaAs. In general, synthesisprocesses of single crystalline group III-V are very costly because ofsubstantial complications involved in epitaxial growth of group III-Vsingle crystalline compound semiconductors. Typical conversionefficiencies of group III-V compound semiconductor based photovoltaiccells, as these types of photovoltaic cells are intended to be, can beas high as ˜34% when combined with germanium substrates, another veryexpensive material [King R R, Fetzer C M, Colter P C, Edmondson K M, LawD C, Stavrides A P, Yoon H, Kinsey G S, Cotal H L, Ermer J H, Sherif RA, Karam N H. Lattice-matched and metamorphic GaInP/GaInAs/Geconcentrator solar cells. Proceedings of the World Conference onPhotovoltaic Energy Conversion (WCPEC-3), Osaka, May 2003; to bepublished.], usually more than 10 times as expensive as the conventionalSi-solar cell.

All photovoltaic cells in the prior art described above, regardless ofwhat materials the cell is made from, essentially fall into one specifictype of structure, which usually limits its power generation capability.Usually a flat pn-junction structure is used in conventional solar cells(FIG. 1A). Shown in FIG. 1A is a photovoltaic cell comprising a thickp-type semiconductor layer 101 and a thin n-type semiconductor layer 102formed on an electrically conductive substrate 100. A pn-junction 103 isformed at the interface between the p-type semiconductor layer 101 andthe n-type semiconductor layer 102. Incident light 104 entering the cellgenerates electron-hole pairs after being absorbed by the p- and alson-type semiconductor layers 101 and 102. The incident light 104generates electrons 105 e and holes 105 h in the region near thepn-junction 103 and also electrons 106 e and holes 106 h in the regionfar from the pn-junction 103. The photogenerated electrons 105 e and 106e (and holes) (hereafter considering only electronics, i.e. minoritycarriers in p-type semiconductors, although the same explanation isapplicable for holes, minority carriers in n-type semiconductors)diffusing toward the pin-junction 103 and entering the pn-junction 103contribute to photovoltaic effect. The two key factors thatsubstantially impact the conversion efficiency of this type ofphotovoltaic cell are photo carrier generation efficiency (PCGE) andphoto carrier collection efficiency (PCCE).

The PCGE is the percentage of photons entering a cell which contributeto the generation of photo carriers, which needs to be, ideally, 100%.On the other hand, the PCCE is the percentage of photogeneratedelectrons 105 e and 106 e that reach the pn-junction 103 and contributeto the generation of photocurrent. For a monochromatic light, a PCGE of˜100% can be achieved by simply making the p-type layer 101 thicker;however, electrons 106 e generated at the region far away from thepr-junction 103 cannot be collected efficiently due to many adverserecombination processes that prevent photogenerated carriers fromdiffusing into the pn-junction 103. Thus, the basic structure of currentphotovoltaic cells has its own limitation on increasing the conversionefficiency. As the minority carriers travel through the semiconductors,the longer the life-time, the less recombination, which increases theconversion efficiency. Usually, a thicker and higher quality wafer isused to increase the conversion efficiency of the conventional solarcell. However, this makes the solar cell costly and heavier. In additionto increasing the collection efficiency, the absorption of a wide rangeof the solar spectrum will also increase the conversion efficiency. Itis highly desirable to have the solar cell structure in which (a) theincrease of the PCCE is independent of the substrate thickness and (b)the ability to absorb a wide range of the solar spectrum is possible.

FIG. 1B shows typical monochromatic light intensity behavior 108 insidethe semiconductor. As illustrated in FIG. 1B, the light intensitybehavior 108 inside the bulk semiconductor is exponential. The lightintensity p at certain depth x can be expressed as p(x)=P_(o)exp(−αx),where P_(o) is the peak intensity at the surface and α is the absorptionco-efficient of the semiconductor in which light is entering. Carriers(not shown here) generated due to light flux 112 absorbed by thepn-junction 103 is only drifted by the junction field and can becollected efficiently, whereas, carriers 106 e and 106 h generated dueto absorption of light-flux 110 by semiconductor region 101 are diffusedin all directions. Only those carriers 105 e and 105 h which aregenerated closer (a distance equal to or less than the diffusion-lengthof the semiconductor) to the pn-junction 103, can be collected. Thosecarriers 106 e and 106 h which are generated far away (a distance longerthan the diffusion-length of the semiconductor) from the pn-junction 103are recombined and lost. The light flux 112 is usually lost either byleaving out or being absorbed by the substrate.

Both PCGE and PCCE are largely dependent on the material and structureof the photovoltaic cells. Today's photovoltaic cells are structured insuch a way that (a) wide ranges of the solar spectrum cannot be absorbeddue to material limitations, and (b) PCCE is low due to its inherentstructure. For example, the typical conversion efficiency of today'scrystal-Si based solar cell is ˜18%. Wavelengths of the solar spectrumspread from <0.1 μm to 3.5 μm, but Si can only absorb ˜0.4 μm to 0.9 μmof light. ˜50% of light belonging to the solar spectrum cannot beabsorbed by Si, due to its inherent material properties. The remaining32% is lost due to (i) recombination of photogenerated carriers and (ii)loss of light, which is represented by 112 in FIG. 1B; these two factorsare structure dependent. If we could reduce these two factors, ˜50%conversion efficiency could be achieved, even in a Si-based solar cell.If we could capture different wavelengths of light belonging to thesolar spectrum by utilizing different material systems or nano-materialsystems, we could increase the conversion efficiency ideally to 100%.Furthermore, if the solar cell (photovoltaic cell) detection capabilitycould be extended to the infrared-spectrum, then the cell could produceelectrical energy not only during the day (while sun is present), butalso at night (hereafter defined by when the sun is not out).Additionally, today's solar cell material is not highlyradiation-tolerant. Specifically, in space applications, photovoltaiccells should be highly radiation tolerant and have structure andmaterial systems which can generate high-power per unit area. In orderto increase the conversion efficiency (ideally to 100%), it would bedesirable to have photovoltaic cell structures which have (a) largersurface area to volume ratios to capture all the photons (at specificwavelength) entering the cell, (b) a pn-junction that is located asclose to the photo absorption region as possible, and (c) photoresponses at different spectrums in order to efficiently cover a widerange of spectrums of light that enter a photovoltaic cell. It would befurther desirable to have solar cells which could generate electricpower in both day and night.

In addition to conversion efficiency, cost-effective manufacturing isanother important factor which needs to be taken into consideration. Intoday's solar cell, the high-cost is one of the main concerns inaddition to the issue of achieving low conversion efficiencies. It isfound that more than 93% of solar cells are silicon (Si) based, meaninga silicon (Si) wafer is the base material, and the rest are thin-filmbased solar cells. In manufacturing Si-based solar cells, more than 85%of the cost comes from the Si wafer cost, the remaining comes from otherprocessing costs. It is highly desirable to reduce the wafer cost and atthe same time increase the conversion efficiency.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide structures ofphotovoltaic cells, which have high power generation capability per unitarea, with respect to the conventional counterparts, mentioned as theprior art.

Accordingly, it is an object of this invention to reduce therecombination of carriers and increase the absorption of light, whicheffectively increases the photogenerated carriers.

Accordingly, it is an object of this invention to increase theabsorption bandwidth of the solar spectrum in order to increase theconversion efficiency.

It is an object of this invention to provide solar cell structures basedon nano-scaled blocks structures which are formed on the base substrate.The pn- or Schottky junctions are formed with nano-blocks, generatingbuilt-in potential by which photogenerated electrons and holes are sweptaway, leading to photovoltaic effect.

It is an object of this invention to provide solar cell structures basedon nano-blocks, such as rods or wires or nanoparticles, formed on thesupporting substrate or formed on the electronic materials which areformed on the base substrate. The pn- or Schottky junctions are formedwith nano-blocks, generating built-in potential by which photogeneratedelectrons and holes are swept away, leading to photovoltaic effect.

According to this invention, the supporting substrate can be Si, CdTe,Cu, GaAs, InP, GaN, glass, polymer, ceramics, Ge, C, ZnO, BN, Al₂O₃,AlN, Si:Ge, CuInSe, II-VI, III-V, etc.

It is an object of this invention to have electronic materials on whichnano-blocks (rods, wires, or tube) or nanoparticles can be formed andsaid electronic materials can be formed on the base substrate, madefrom, for example, Si, Ge or glass to lower the cost.

It is an object of this invention to provide structures of photovoltaiccells which can capture most of the wavelengths belonging to the solarspectrum and can provide >80% conversion efficiency.

It is an object of this invention to provide structures of photovoltaiccells which can generate electric power in both day and night.

It is an object of this invention to provide low-cost manufacturingprocesses for manufacturing the novel photovoltaic cells.

This invention proposes to achieve >60% conversion efficiency utilizingSi-materials and >80% conversion efficiency for other materials. Themain advantage of these inventions are that today's matured processtechnologies allow fabrication of the photovoltaic cell which has powergeneration capabilities a few orders or more greater than that ofconventional photovoltaic cells.

Other objects, features, and advantages of the present invention will beapparent from the accompanying drawings and following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in conjunction with theappended drawings wherein:

FIG. 1A is the schematic showing the cross-sectional view of aconventional photovoltaic cell structure. This is the explanatorydiagram showing the prior-art of today's photovoltaic cell. FIG. 1Bshows the light intensity behavior inside prior-art semiconductormaterials.

FIG. 2A is the schematic showing the nanoparticle structures whichillustrate the benefits of achieving a significantly high ratio ofsurface area to volume used in the simulation. FIG. 2B is the simulationresults, showing the ratio of the surface area T to the volume A as afunction of diameter of the nanoparticle d with the gap between theparticles gas the parameter.

FIGS. 3A and 3B are the schematics showing the cross-sectional views ofa photovoltaic cell structure comprising the nanoparticles in the firstembodiment, in accordance with the present invention.

FIGS. 4A, 4B, and 4C are the schematics showing the cross-sectionalviews of photovoltaic cell structures comprising nanoparticles in thesecond embodiment, in accordance with the present invention.

FIGS. 5A and 58 are the schematics showing the cross-sectional views ofa photovoltaic cell structures comprising layers of nanoparticles ofdifferent sizes and types in the third embodiment, in accordance withthe present invention.

FIG. 6 is the schematic showing the cross-sectional view of aphotovoltaic cell structure based on the nanoparticles layer and thecylindrical structure, in the fourth embodiment, in accordance with thepresent invention.

FIG. 7 is the schematic showing the cross-sectional view of aphotovoltaic cell structure based on the nano-scaled cylindrical blocks,in the fifth embodiment, in accordance with the present invention.

FIG. 8 is the schematic showing the cross-sectional view of aphotovoltaic cell structure based on the nano-scaled blocks embeddedinto the polymer, in the sixth embodiment, in accordance with thepresent invention.

FIGS. 9A, 9B, 9C, and 9D are the schematics showing the fabricationprocess flow for flexible photovoltaic cells comprising a nanoparticlelayer as the absorption layer, in the seventh embodiment, in accordancewith the present invention.

FIGS. 10A, 10B, 10C, and 10D are the schematics showing an alternatefabrication process flow for flexible photovoltaic cells comprisingnanoparticles embedded into the polymer layer, in the eighth embodiment,in accordance with the present invention.

FIGS. 11A, 11B, 11C, 11D, and 11E are the schematics showing analternate fabrication process flow for photovoltaic cells comprisingnano-scaled wires/blocks formed on the dielectric or semiconductorsubstrate, in the ninth embodiment, in accordance with the presentinvention.

FIGS. 12A and 12B are the schematics showing the formation of thesemiconductor-metal nanoparticles, as required by FIG. 11 forfabricating the photovoltaic cell.

DETAILED DESCRIPTION

According to the present invention it is our object to provide severalphotovoltaic cell structures that increase the surface area, which wouldincrease the junction area, and also cover a wide range of the solarspectrum in order to increase conversion efficiency to as high as >60%.Before proceeding to give the detailed explanation of the photovoltaiccell structures and their manufacturing, simulation results are given toshow the benefits of increasing the surface area and using thenanoparticles in the photovoltaic cell.

FIGS. 2A and 21 are the schematics representing the layer 202 formedwith the nanoparticles. The layer 202 represents the absorption layer inthe photovoltaic cell. FIG. 2B shows the simulation results as functionof the nanoparticle diameter d with gap g between two nanoparticles asthe parameter in FIG. 2A, n×n number of nanoparticles are arranged ina×a sized layers with thickness t. T/A is the ratio of the surface areaof the total nanoparticles T to the volume (a×a×t=A). It is assumed thatif we could make the n×n number of nanoparticles 204 on the a×a-areasurface. Increase of the ratio indicates the increasing increment of thesurface area Tin the proposed cell, as compared to the conventionalphotovoltaic cell, which is usually flat. As depicted in FIG. 2B, byreducing the gap g and the nanoparticle diameter d, the ratio increasessignificantly. For example, if the particle gap g is kept to 5 nm andthe particle diameter d is kept to 5 nm, over 750 times the surface areacan be achieved as compared to a conventional 5 cm×5 cm photovoltaiccell.

According to the preferred embodiment illustrated in FIGS. 3A and 3B, aphotovoltaic cell comprising a nanoparticle based layer 301 acts as theabsorption layer, sandwiched by top metal contact 303 a and bottom metalcontact 303 b, formed on the supporting substrate 300. The nanoparticles302 can be formed as the layer (not shown here) acting as the absorptionlayer. The nanoparticles 302 can be embedded into the electronicmaterial 304, creating electrical conduction. The electronic material304 can be semiconductor material or conductive polymer material, whichmay create the junction (n or p type junction) with the nanoparticles302. A single or plurality of layers may be require to make the junction(not shown here). The electronic material 304 can be a separate materialor electronic materials of p or n type. The electronic material 304 andthe supporting substrate 300 are further electrically connected toelectrodes 303 a and 303 b, respectively. The electrodes 303 a and 303 bare intended to serve as common electrodes which connect allnanoparticles 302. The electrode 303 a is provided for the electronicmaterial 304 or junction. The electrode 303 a can be transparent (notshown here) and can be formed on the electronic material 304 orjunction. The interface between the nanoparticle 302 and the electronicmaterial (or junction) 304 form pn- or Schottky junctions where built-inpotential for both electrons and holes is generated. The nanoparticlescould be made of any kind of semiconductor material of a few nanometersin size, which could create the quantum confinement effect. Absorptionof a broad range of the solar spectrum is possible by utilizingdifferent sizes and different semiconductor nanoparticles. Substrate 300could be semiconductor, glass, or polymer.

According to this invention, alternatively the nanoparticle based layer301 can be formed various ways as shown in FIGS. 4A, 4B, and 4C. Asshown, the nanoparticle based layer 401 (401 a, 401 b, and 401 c) actingas the absorption layer, can be formed by stacking nanoparticles 402which are either of same size (402 a), different size (402 b), ordifferent type (402 c), Alternatively, the nanoparticles 402 (402 a, 402b, and 402 c) can be embedded into the electronic conducting material(not shown here). In way of an example not way of limitation, thenanoparticles 402 can be made of semiconductor material (n- or p-type)and the electric conduction material (not shown here) that is on orsurrounds the nanoparticles 402 can be made of p-type semiconductor.Incident light 406 (FIG. 4C) enters the photovoltaic cell through eitherthe electrode 403 a or on the material or junction substrate 400. (InFIG. 4C, the incident light 406 enters the photovoltaic cell through theelectrode 403 a). The incident light 406 travels through thenanoparticle based layer 401, and the substrate 400. The light absorbedby both nanoparticles 402 and the electronic material (not shown here)generates numerous electrons (not shown here). Each nanoparticle 402helps form the junction all over the surface of the electronic material(not shown here), and helps to collect the carriers without allowingrecombination. Utilizing the nanoparticles 402 helps to collect thecarriers with less or even no recombination by increasing the junctionarea. It should be pointed out that electrons are apparently generatedall over the region along the thickness of the electric material and thenanoparticles 402. It also should be pointed out that holes areapparently generated all over the region along the thickness of theabsorption layer 401.

Unlike a conventional solar cell, the solar cells shown in FIGS. 4A, 4B,and 4C have junctions all over the nanoparticles 402, which create anequivalent to multiple junctions across the thickness of the absorptionlayer 401. Increasing the thickness would help to absorb photon fluxperpendicular to the front contact. As the particle size is nano-scaled,absorption of a broad range of the spectrum can be possible and almostall carriers can be collected, as the junctions are formed a fewnanometers apart. It is apparent that utilizing the solar cell shown inFIGS. 3 and 4 can (i) reduce the recombination, (ii) absorb all photoflux, and (iii) cover a broad range of the spectrum, thereby increasingthe conversion efficiency.

According to the preferred embodiment illustrated in FIGS. 5A and 5B, aphotovoltaic cell comprising a plurality of absorption layers 501 basedon nanoparticles of different types 508, 510, and 512 are formed on thebottom contact 503 b which is on the substrate 500. The only differencehere, when compared to FIGS. 3 and 4, is that the absorption layer 501consists of multiple layers of nanoparticles of different type anddifferent sizes 508, 510, 512. Similar to FIGS. 3 and 4, the surfacearea of the junction for receiving light 506 is also increased whichcauses a reduction of the photogenerated carrier recombination and theabsorption of all photo-flux incident on the surface, thereby increasingthe conversion efficiency. Note here that by utilizing multiple layersof nanoparticles of different type and sizes, absorption of the fullsolar spectrum can be possible and, ideally, the conversion efficiencycan be increased to 100%.

Apparent advantages of this invention over conventional photovoltaiccells are directly associated with the fact that, unlike conventionalphotovoltaic cells, multiple discrete junctions are created forcollecting all photogenerated carriers created in the absorption layer501, regardless of where they are generated. According to thisinvention, recombination can be eliminated (ideally) and all photon fluxcan be absorbed (ideally), and the conversion efficiency can be ˜100%and still >50% when using Si. Conventionally, as explained in thedescription of the prior art shown in FIG. 1, a conventionalphotovoltaic cell, where electrical-junctions are perpendicular to thedirection to which incident light travels, the photogenerated carriersgenerated in the region far away from electrical-junctions need todiffuse much longer distances (diffusion-length) than thosephotogenerated carriers generated near the junctions, thus they have agreater chance to recombine without contributing to photovoltaiceffects. Therefore, in this invention, PCCE is expected to be muchhigher than that of conventional photovoltaic cells. In addition, it isevident that the total effective area that contributes to thephotovoltaic effect in this invention can be increased significantly bya few orders (>3000) considering a 5 nm diameter with a gap of 0 nm fora 5 cm×5 cm size cell.

According to this invention, in way of an example not way of limitation,the supporting substrate 500 can be ceramics, glass, polymer or any kindof semiconductor on which transparent or nontransparent metal contact503 b is made. Alternatively, supporting substrate 500 can be metalwhich also acts the metal contact. For this case, copper, stainlesssteel, Aluminum, or alloyed metal can be used. According to thisinvention, the nanoparticles 508, 510, 512 can be any kind ofsemiconductor or compound semiconductors, having absorption capabilitiesin the desired spectrum region. By utilizing the quantum confinementeffect, which is dependent on the size of the nanoparticle, theabsorption range can be extended in the blue and red-shift of theirparent bulk type materials. For nanoparticles, Si, Ge, InP, GaAs, CdSe,CdS, ZnO, ZnTe, ZnCdTe, CuInSe, CuSe, InGaAs, etc. can be used.

According to this invention, in a way of an example not way oflimitation, the nanoparticles 508, 510, 512 can be stacked to form theabsorption layer 501. Alternatively, the electronic conduction materialcan be used to embed the nanoparticles 508, 510, 512. The electricalconduction material can be the sol-gel layer or any conductive polymer.The top metal contact 503 a can be transparent or non-transparent metal.Indium-tin-oxide (ITO) can be used as the transparent metal contact.Alternatively, the electrical conduction layer can be formed onto theabsorption layer 501 to create the junction.

In an alternative preferred embodiment shown in FIG. 6, photovoltaiccell comprises a substrate 600 with three-dimensional protrusionswherein the cross-sectional shape of the three-dimensional protrusionsis cylindrical, absorption layer 601 based on the nanoparticles 602,electrical conduction layer 604, and top and bottom metal contacts 603 aand 503 h, respectively. The supporting substrate 600 can be flexible orrigid substrate formed by molding, casting, or etching. The substrate600 can be polymer, semiconductor or ceramic, or glass type material.Alternatively, supporting substrate 600 can be metal, which can alsowork as the bottom contact (not shown here). The nanoparticles 602 andthe electrical conduction material 604 form the absorption layer 601.The incident beam 606 is absorbed by both the electrical conductionmaterial 604 and the nanoparticles 602, which generate the electrons(and holes), which are in turn collected by the metal contacts 603 a and603 b. The junction created by the electrical conduction material 604and nanoparticles 602 helps sweep away the generated carriers to thecontacts 603 a and 603 b. The electronic material 604 is furtherelectrically connected to electrodes 603 a and 603 b, which are intendedto serve as common electrodes that connect all cylindrical shapedelectrical junctions. The electrode 603 a is on the electronic material604. The interface between the nanoparticles 602 and the electronicmaterial 604 form pn- or Schottky junctions, which can be assumed to bemultiple discrete junctions inside the conduction material 604.

Photogenerated electrons in the electronic material 604, made of p- andn-type semiconductor or conductive polymer, then diffuse toward thejunction created by conduction material 604 and nanoparticles 602. Atthe junctions, the diffused electrons are swept away by built-inpotential, thus photovoltaic effects set in. Common advantages alreadydescribed for the photovoltaic cells in FIGS. 3, 4, and 5, can beachieved in this embodiment as well. The only differences in FIG. 6 arethat the supporting substrate 600 shape is cylindrical and single ormultiple layered nanoparticles are used to increase the combined surfaceto volume ratio significantly.

According to this invention, in way of an example not way of limitation,the supporting substrate 600 can be semiconductors such as Ge, GaAs,GaN, InP, GaN, CdTe, or ZnO or polymer or metal.

In the preferred embodiment shown in FIG. 7, a photovoltaic cellcomprises a plurality of nanometer(s) scaled cylinders 714 which areelectrically connected to a substrate 700. The nano-meter(s) scaledcylinders 714 are surrounded by an electronic material 716, havingmetallic electrical conduction and forming the junction in the interfaceof 716 and 714. The electronic material 716 and the supporting substrate700 are further electrically connected to electrodes 703 a and 703 b.The electronic conduction material 716, used to make the junction withnano-meter (s) scaled rods/wires 714, is electrically isolated with thebottom contact 703 b by dielectric 718. The planarization dielectric(polymer) 720 is on which the metal electrical contact 703 a is formed.The electrode 703 a is intended to serve as a common electrode thatconnects all rods 714. The electrode 703 a is provided for theelectronic material 716. The interface between the nanometer(s)-scalerods 714 and the electronic material 716 form pn- or Schottky junctions,thus there are pn- or Schottky junctions outside of thenanometer(s)-scale rods 714.

In an alternative preferred embodiment shown in FIG. 8, a photovoltaiccell comprises a plurality of nanometer(s)-scaled rods (or wires) 822embedded into the electrical conduction material 804 acting as theabsorption layer 801, which is electrically connected to a substrate800. The micrometer(s) or nanometer(s)-scaled rods (or cylinders) 822are surrounded by an electronic material 804 having electricalconduction. The electronic material 804 can be a separate material inwhich nanometer-scaled rods or wires 822 are embedded. The electronicmaterial 804 and the supporting substrate 800 are further electricallyconnected to electrode 803 a on the non-substrate side and electrode 803b on the substrate side. The interface between the nanometer(s)-scalerods 822 and the electronic material 804 form junctions, thus creatingbuilt-in-potential for collecting photogenerated carriers andtransferring them to the electrodes. The main difference between thephotovoltaic cell shown in FIG. 8 and others of FIGS. 3, 4, 5, and 6, isthat the nanometer(s) scaled rods or wires 822 are separatelysynthesized and are embedded into the electronic material 804 formed onthe supporting substrate 800. For example, the nanorods or wires 822 canbe semiconductors like, Si, Ge, InP, GaAs, Cds, Cdse, CdTe, ZnO, Znse,ZnS, etc., or combinations thereof, separately synthesized either byepitaxial growth in a vacuum deposition system or chemically formed fromthe solutions.

According to this invention, in way of an example not way of limitation,the supporting substrate 800 can be Ge, GaAs, GaN, CdTe, ZnO, Cu, Al₂O₃,AlN, glass, polymer, metal, etc. The electronic material 804 can beconductive polymer or a sol-gel based semiconductor. Note here that useof the nanometer-scaled rods or wires 822 helps not only increase thejunction area but also helps transfer the generated carriers to theelectrodes 803 a and 803 b before recombination. The rods or wires 822inside the conductive polymer help to transfer the carriers through therods. By varying the size (diameter) of the nanometer scaled rods (orwires) 822, a quantum confinement effect can be created and therebyabsorption of broad spectral ranges can be possible.

FIGS. 9A, 9B, 9C and 9D are the schematics showing the fabricationprocess of the flexible photovoltaic cell based on nanoparticlescontained in the absorption layer, according to this invention, whereinthe similar numerals in FIG. 9 represents the similar parts in FIGS. 3,4, 5, and 6, so that similar explanations are omitted. According to thisinvention, supporting substrate 900 can be semiconductor such as Si, Ge,GaAs, InP, etc., polymer, or glass. The metal 903 b acting as thecontact in the photovoltaic cells, is formed onto the substrate 900. Themetal 903 b can be pressed and a rolling technique can be used,utilizing the roller 930. To make a firm connection with the substrate,epoxy or glue can be used. The absorption layer 901, comprising thenanoparticles embedding into the conducting polymer, is formed on themetal contact 903 b. To form the absorption layer 901, spin coating orpress and rolling techniques can be used. Alternatively, a printingtechnique or ink-jettable technique can also be used to form theabsorption layer 901. With proper heat treatment, the top contact 903 acan be formed. It is noted here that the instead of using the substrate900, alternatively the absorption layer 901 can be directly formed ontothe metal 903 b by using the printing or ink-jettable technique. Theadvantages of this process are that low-cost and high efficiencyflexible photovoltaic cells can be fabricated.

FIGS. 10A, 10B, 10C, and 10D are the schematics showing the fabricationprocess of the photovoltaic cell based on nanoparticles contained in theabsorption layer 1001, according to this invention, wherein the similarnumerals in FIG. 10 represent similar parts in FIGS. 3, 4, 5, and 6, sothat similar explanations are omitted. According to this invention, alayer of nanoparticles 1002 is formed on the substrate 2000 by chemicalsynthesis. A thin layer (not shown here) may be required in betweensubstrate 2000 and the nanoparticle 1002, to increase the stickingcoefficient. The layer of nanoparticles 1002 then transferred to theforeign substrate 1000 with metal electrode 1003 b, on which thephotovoltaic cell is to be made. The transfer of the nanoparticles 1002to the foreign substrate 1000, can be done by heat or imprintingtreatment. According to this invention, foreign substrate 1000 can be asemiconductor such as Si, Ge GaAs, InP, etc., polymer, or glass.Alternatively, the layer of nanoparticles 1002 can be directlytransferred to the metal which could act as the substrate and also asthe contact for the photovoltaic cell. Alternatively, without using thetransferring process described in FIGS. 10A and 10B, the nanoparticle1002 layer can be formed using printing or ink-jettable printingprocesses. The absorption layer 1001 comprising the nanoparticles 1002embedding into the conducting polymer 1004 is formed on the metalcontact 1003 b. To form the absorption layer, spin coating or press androlling techniques, as mentioned earlier, can be used. Alternatively,printing or ink-jettable techniques can also be used to form theabsorption layer 1001. After proper heat treatment, the top contact 1003a is formed. It is noted here that the instead of using the substrate1000, alternatively the absorption layer 1001, can be directly formedonto the metal 1003 b, using printing or ink-jettable techniques. Theadvantages of this process are that low-cost and high efficiencyflexible photovoltaic cells can be fabricated.

FIGS. 11A, 11B, 11C, 11D, and 11E are the schematics showing thefabrication process of the photovoltaic cell based on nanoscaled rods orwires 1114 formed on the supporting substrate 1100, according to thisinvention, wherein the similar numerals in FIG. 11 represent similarparts in FIG. 7, so that similar explanations are omitted. According tothis invention, nano-meter sized metal-semiconductor compounds 2100 areformed on the substrate 1100 with the metal electrode 1103 b. Themetal-semiconductor compounds can be formed either on the substrate 1100by chemical synthesis, or transferred to the substrate 1100 byimprinting or heat treatment processes. According to this invention, themetal-semiconductor compounds 2100 have semiconductor and metal attachedtogether. The semiconductor portion is attached to the substrate and themetal portion acts as the catalytic and is on top of the semiconductor.They are periodically organized onto the substrate 1100 with metalcontact 1103 b by imprinting or lithography processes. With chemicalvapor deposition, the nano-scaled rods 1114 are formed. A dielectriclayer, such as silicon-oxide or silicon nitride layer 1118, is used toisolate the junction formed by the electronic material 11116 andnano-rods 1114.

To make the pn-junctions of dissimilar type (p or n), electronicmaterial 1116 is used. For example, if the rod-material 1114 is n-typeSi, then p-type Si is to be formed as the electronic material 1116. Thiscan be formed by the diffusion of p-dopants into the n-type substrate.The interface of 1114/1116 forms the junction which has thebuilt-in-potential to create the photovoltaic effect on a large surfacearea. Finally, a passivation or conformal layer of dielectric or polymer1120 is formed on the electronic material 1116 after proper chemicalmechanical processes. The final stage is to make the planarization usinginsulator layer 1120 and contact 1113 a.

FIGS. 12A and 12B are the schematics showing the formation of thesemiconductor-metal interface onto the desired substrate 1200, forexample glass for fabricating the photovoltaic cell as explained in FIG.11, according to this invention, Here, the nano-silicon particles areformed uniformly onto the desired substrate 1200 with the metal contact1203 b. Nano-silicon particles made separately by chemical synthesis canbe used for this purpose. This is followed by the nanometal attachmenton to the nano-silicon particles, as shown in FIG. 12B. The nanometalonto the nano-silicon acts as the catalytics for forming the nano-rods,as explained in FIG. 11B.

According to this invention, the absorption layer formed by theelectronic material and the nanoparticles or nanometer(s) scaled rods,explained in FIGS. 3 thru 11 is a single layer used to form thejunctions. By varying the diameter of the nanoparticles or the nano-rodsused in the absorption layer, absorption of a wide range of the solarspectrum is possible, which increases the power generation.

According to this invention, as explained in FIGS. 3 thru 11, thenanoparticles or nano-rods are used to increase the junction area, sothe junction can be extended closer to the region where thephotogenerated carriers are formed. The 3-dimensional structures can beused as part of the electronic material to form the junctions. Thisstructure can be formed utilizing a suitable substrate. The substratecan be used to form the structure where junctions are formed, utilizingthe other electronic materials formed on to the 3D structures.

According to this invention, the nanoparticles or rods could be GaNmaterials (n or p type) and the dozens of materials could beIn_(1-x)Ga_(x)N (p or n type, opposite the GaN rods). By increasing theGa contents, the band-gap of InGaN can be increased to ˜3.4 eV, which isthe same as that of GaN. By increasing the in content in InGaN, the bandgap can be reduced to ˜0.65 eV. Photons with less energy than the bandgap will slip right through. For example, red light photons are notabsorbed by high-band-gap semiconductors, while photons with an energyhigher than the band gap are absorbed, for example, blue light photonsin a low-band-gap semiconductor—their excess energy is wasted as heat.

According to this invention, alternatively the nanoparticles or rodscould be III-V based materials (n or p type), for example InP, and thedozens of the materials could be III-V based material likeIn_(1-x)Ga_(x)As (p or n type, opposite the InP rods). In this case, byadjusting the In contents, the band gap can be tuned and thereby a widespectrum of the solar energy can be absorbed.

According to this invention, alternatively the nanoparticles or rodscould be II-V based materials (n or p type), for example CdTe, and thedozens of the materials could be II-VI based material like CdZnS (p or ntype, opposite the CdTe rods) or Zn(Cd)Te/ZnS based materials. In thiscase, by adjusting the Zn contents, the band gap can be tuned andthereby a wide spectrum of the solar energy can be absorbed.

According to this invention, alternatively the nanoparticles or rodscould be Si or amorphous Silicon materials (n or p type) and the dozensof the materials could be Si: Ge alloy (p or n type, opposite the Sirods). In this case, by adjusting the Ge contents, the band gap can betuned and thereby a wide spectrum of solar energy can be absorbed.

According to this invention, alternatively the nanoparticles or rodscould be Si, InP, or CdTe (n or p type) and dozens of differentmaterials could make the junction with the rods (wires or tubes) andeach type of material would have a specific band gap for absorbing aspecific range of the solar spectrum. In this way a wide range of thesolar spectrum can be absorbed, and by increasing the junction area (dueto use of the rods, wires, or tubes), the electrical power generationcould be increased tremendously (50 times and beyond).

According to this invention, the nanoparticles or nanometer(s)-scalewires, mentioned in the preferred embodiments, can be any kind ofelectronic materials, covering semiconductor, insulator, or metal.

According to this invention, the nanometer sized nanoparticles or rodscan be made from semiconductors such as Si, Ge, or compoundsemiconductors from III-V or II-VI groups. As an example for rods,wires, or tubes, InP, GaAs, or GaN III-V compound semiconductors can beused and they can be made using standard growth processes, for example,MOCVD, MBE, or standard epitaxial growth. According to this invention,the self-assembled process can also be used to make wires, rods, ortubes and their related pn-junction in order to increase the junctionarea. These rods, wires, or tubes can be grown on the semiconductors(under same group or others), polymers, or insulators. Alternatively,according to this invention, these rods, wires, or tubes, can betransferred to the foreign substrate or to the layer of foreignmaterial. The foreign substrate or the layer of material can be anysemiconductor such as Si, Ge, InP, GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe,HgCdTe, etc. The substrate can also cover all kinds of polymers orceramics such as AlN, Silicon-oxide, etc.

According to this invention, the nanometer sized nanoparticles or rods,based on an II-VI compound semiconductor can also be used. As an exampleCdTe, CdS, Cdse, ZnS, or ZnSe can be used, and they can be made usingstandard growth processes, for example, sputtering, evaporation, MOCVD,MBE, or standard epitaxial growth or chemical synthesis. According tothis invention, the self-assembled process can also be used to makenanoparticles or wires, and their related pn-junctions to increase thejunction area. These rods, wires, or tubes can be grown on thesemiconductors (under same group or others), polymers, or insulators.Alternatively, according to this invention, these rods, wires, or tubes,can be transferred to the foreign substrate or to the layer of foreignmaterial. The foreign substrate or the layer of material can be anysemiconductor such as Si, Ge, InP, GaAs, GaN, ZnS, CdTe, CdS, ZnCdTe,HgCdTe, etc. The substrate can also cover all kinds of polymers orceramics such as AlN, Silicon-oxide, etc.

According to this invention, the nanometer sized rods, wires, or tubescan be made from carbon type materials (semiconductors, insulators, ormetal like performances), such as carbon nano-tubes, which could besingle or multiple layered. They can be made using standard growthprocesses, for example, MOCVD, MBE, or standard epitaxial growth.According to this invention, the self-assembled process can also be usedto make wires, rods, or tubes and their related pn-junction in order toincrease the junction area. These rods, wires, or tubes can be grown onthe semiconductors (under same group or others), polymers, orinsulators. Alternatively, according to this invention, these rods,wires, or tubes, can be transferred to the foreign substrate or to thelayer of foreign material. The foreign substrate or the layer ofmaterial can be any semiconductor such as Si, Ge, InP, GaAs, GaN, ZnS,CdTe, CdS, ZnCdTe, HgCdTe, etc. The substrate can also cover all kindsof polymers or ceramics such as AlN, Silicon-oxide, etc.

Whereas many alterations and modifications of the present invention willno doubt become apparent to a person of ordinary skill in the art afterhaving read the foregoing description, it is to be understood that theparticular embodiments shown and described by way of illustration are inno way intended to be considered limiting. Therefore, reference to thedetails of the preferred embodiments is not intended to limit theirscope.

Although the invention has been described with respect to specificembodiments for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodification and alternative constructions that may occur to one skilledin the art which fairly fall within the basic teaching here is setforth.

The present invention is expected to be found practically useful in thatthe novel photo-voltaic cells have higher power generation capability(25 times and beyond) when compared with that of the conventional cells.The proposed invention can be used for fabricating wide solar panels forboth commercial and space applications.

What is claimed is:
 1. A photovoltaic cell comprising: a substrate withthree-dimensional protrusions, wherein the cross-sectional shape of thethree-dimensional protrusions is cylindrical; a first electrode disposedon the substrate; wherein the first electrode coating thethree-dimensional protrusions and wherein the first electrode conformsto the contour of the three-dimensional protrusions; an absorption layerdisposed on the first electrode; and a second electrode disposed on theabsorption layer, wherein the absorption layer comprises: a layer of aplurality of nanoparticles, wherein the nanoparticles are semiconductorof p- or n-type; and wherein the layer of a plurality of nanoparticlesis disposed on the first electrode; a layer of continuous electronicmaterial disposed upon the layer of the plurality of nanoparticles andelectrically connected to the first electrode or the second electrode,wherein the layer of continuous electronic material is a semiconductorof the opposite conductivity type as the layer of a plurality ofnanoparticles; wherein the layer of a plurality of nanoparticles and thelayer of continuous electronic material forming cylindrical shapedelectrical junctions.
 2. The photovoltaic cell of claim 1, wherein thelayer of a plurality of nanoparticles are stacked into more than onelayer.
 3. The photovoltaic cell of claim 1, wherein the layer of aplurality of nanoparticles is of different material or different size.4. The photovoltaic cell of claim 2, wherein each layer of a pluralityof nanoparticles comprises nanoparticles of a different material ordifferent size from at least one other of the layers of nanoparticles.5. The photovoltaic cell of claim 1, wherein the nanoparticles are rods,tubes, wires, or spheres.
 6. The photovoltaic cell of claim 1, whereinthe layer of a plurality of nanoparticles are comprised of materialselected from the group consisting of Si, Ge, InP, GaAs, CdSe, CdS, ZnO,ZnTe, ZnCdTe, CuInSe, CuSe, InGaAs, and any combination thereof.
 7. Aphotovoltaic cell comprising: a substrate with three-dimensionalprotrusions, wherein the cross-sectional shape of the three-dimensionalprotrusions is cylindrical; a first electrode disposed on the substrate;wherein the first electrode coating the three-dimensional protrusionsand wherein the first electrode conforms to the contour of thethree-dimensional protrusions; an absorption layer disposed on the firstelectrode; and a second electrode disposed on the absorption layer,wherein the absorption layer comprises: a layer of a plurality ofnanoparticles, wherein the nanoparticles are semiconductor of p- orn-type; a layer of first continuous electronic material disposed uponthe layer of the plurality of nanoparticles and electrically connectedto the second electrode; and a layer of second continuous electronicmaterial disposed and electrically connected to the first electrode,wherein the layer of plurality of nanoparticles is disposed on the layerof second continuous electronic material; wherein the layer of aplurality of nanoparticles and the layer of second continuous electronicmaterial forming cylindrical shaped electrical junctions; wherein thelayer of first continuous electronic material is a semiconductor of p-or n-type, and wherein the layer of second continuous electronicmaterial is a semiconductor of the opposite conductivity type as thelayer of first continuous electronic material.
 8. The photovoltaic cellof claim 7, wherein the layer of a plurality of nanoparticles arestacked into more than one layer.
 9. The photovoltaic cell of claim 7,wherein the layer of a plurality of nanoparticles is of differentmaterial or different size.
 10. The photovoltaic cell of claim 8,wherein each layer of a plurality of nanoparticles comprisesnanoparticles of a different material or different size from at leastone other of the layers of nanoparticles.
 11. The photovoltaic cell ofclaim 7, wherein the nanoparticles are rods, tubes, wires, or spheres.12. The photovoltaic cell of claim 7, wherein the layer of a pluralityof nanoparticles are comprised of material selected from the groupconsisting of Si, Ge, InP, GaAs, CdSe, CdS, ZnO, ZnTe, ZnCdTe, CuInSe,CuSe, InGaAs, and any combination thereof.
 13. A photovoltaic cellcomprising: a substrate with three-dimensional protrusions, wherein thecross-sectional shape of the three-dimensional protrusions iscylindrical; a first electrode disposed on the substrate; wherein thefirst electrode coating the three-dimensional protrusions and whereinthe first electrode conforms to the contour of the three-dimensionalprotrusions; an absorption layer disposed on the first electrode; and asecond electrode disposed on the absorption layer, wherein theabsorption layer comprises: a layer of first continuous electronicmaterial disposed upon a layer of second continuous electronic material,wherein the layer of second continuous electronic material is disposedupon and electrically connected to the first electrode; wherein thelayer of a first continuous electronic material and the layer of secondcontinuous electronic material forming cylindrical shaped electricaljunctions; and a layer of a plurality of nanoparticles disposed upon thelayer of first continuous electronic material, wherein the layer offirst continuous electronic material is a semiconductor of p- or n-type;wherein the nanoparticles are semiconductor of p- or n-type; and whereinthe layer of second continuous electronic material is a semiconductor ofthe opposite conductivity type as the nanoparticles.
 14. Thephotovoltaic cell of claim 13, wherein the layer of a plurality ofnanoparticles are stacked into more than one layer.
 15. The photovoltaiccell of claim 13, wherein the layer of a plurality of nanoparticles isof different material or different size.
 16. The photovoltaic cell ofclaim 13, wherein each layer of a plurality of nanoparticles comprisesnanoparticles of a different material or different size from at leastone other of the layers of nanoparticles.
 17. The photovoltaic cell ofclaim 13, wherein the nanoparticles are rods, tubes, wires, or spheres.18. The photovoltaic cell of claim 13, wherein the layer of a pluralityof nanoparticles are comprised of material selected from the groupconsisting of Si, Ge, InP, GaAs, CdSe, CdS, ZnO, ZnTe, ZnCdTe, CuInSe,CuSe, InGaAs, and any combination.